This invention relates to apparatus for the exposure of photosensitive material coated on plates and, more particularly, to apparatus for properly registering intelligence bearing transparencies for use in exposing photosensitive materials on plates to a source of actinic radiation.
Much has been done in the field of printed circuit boards over the past few years. Typically a board to be treated is coated with a photosensitive or photo-resist material and exposed to light through a transparent master (artwork) containing the desired circuit layouts. The exposed (or unexposed) regions of the treated material are then chemically treated and dissolved, leaving only the desired pattern on the board which now may be further processed as desired.
Although the chemical processing is by now relatively well known and well developed, a particular problem arises in the registering of the transparent masters and retaining them in position for exposing successive boards to the same circuit patterns. Registration is a particular problem when it is required to expose both sides of a board coated with photosensitive material.
It has been general practice to employ a pair of open frames to support the transparent master, usually glass or plastic, and to secure the master(s) to either or both of the frames by various means including spring clips, screw clamps, strips of pressure-sensitive adhesive or the like. Indicia on the face of each transparent master permit visual alignment of paired masters that are to be used in exposing both sides of a given board. After alignment, the masters are manually secured to the respective frames. The photopolymer coated boards (laminates) are inserted between the masters, exposed to actinic radiation, and removed. While such a procedure may be satisfactory for long production runs in which many duplications are made, in a situation in which one or only a few boards are to be treated after which the master is removed and replaced by a different master, these repetitive procedures incident to installing of the different masters are time-consuming and costly.
One method of securing the masters and coated circuit boards together prior to exposure to light has been to subject the entire region between the frames to a vacuum. While this is proven generally satisfactory, one problem that has arisen is that if any portion of the surface area of the transparent region of the frames becomes uncovered by the masters or other sheet-like material, often the vacuum is destroyed and the sheets not securely held. This requires additional time in that the person installing the masters, must spend considerable time cutting and trimming to be certain of covering all of the vacuum apertures.
Accordingly, it is an object of this invention to obviate many of the disadvantages of the prior art contact exposure devices.
A further object of this invention is to provide an improved apparatus for registering and holding in registry, transparent masters for use in exposing photosensitive materials to images.